1. Field of the Invention
The present invention relates to multibeam longitudinal-interaction electron tubes such as, for example, klystrons or traveling wave tubes.
2. Discussion of the Background
Klystrons, or traveling wave tubes, generally constructed about an axis, comprise several longitudinal electron beams parallel to this axis. These beams are often produced by a common electron gun, fitted with several cathodes, and are connected at the end of travel in one or more collectors. Between the gun and the collector, the beams pass through a body which is a microwave structure at the output of which microwave energy is extracted. This structure may be formed from a succession of resonant cavities and of drift tubes. The electron beams, in order to maintain their long thin shape, are focused by the magnetic field of a focuser which is centered on the main axis and surrounds the microwave structure.
The advantages of multibeam electron tubes are the following: the current produced is higher and/or the high voltage is lower and/or the length is shorter.
For approximately equal performance, the overall size of the tube is generally smaller. The electrical supply and the modulator used are thus simplified and more compact. The efficiency of interaction is better because of the generally lower perveance of each of the beams.
In the case of klystrons, the bandwidth is increased because of the fact that the cavities are charged by a higher current.
Compared with single-beam tubes, one of the main drawbacks is that it is difficult to generate an optimum magnetic focusing field which allows the beams to travel through the microwave structure without appreciable interception by the drift tubes.
In multibeam klystrons, the intercepted current, called the body current, is often about 4 to 8%, whereas it does not exceed 2 to 3% in conventional single-beam klystrons even when the beam is greatly high-frequency-modulated, as is the case with high-efficiency klystrons.
Excessive interception entails not only prohibitive heating, which requires a complex and expensive cooling system, but also poor operation of the tube since expansion, degassing, frequency changes, oscillations, excitation of spurious modes, reflected electrons, ion bombardment and perturbed interaction between the beam and the microwave structure may occur.
This interception is due to the increase in the space charge forces due to the effect of greater density modulation as one approaches the collector, thereby resulting in an increase in the cross section of the beams which consequently come closer to the walls of the drift tubes. It is also partly due to the focuser which inevitably produces a radial magnetic field in the regions where the axial magnetic field varies, that is to say near the gun and the collector. In addition, since the focuser is never perfect, defocusing parasitic magnetic components are produced.
Another important cause of defocusing specific to multibeam tubes is that each beam creates an azimuthal magnetic field which, depending on the configuration of the tube and its mode of operation, runs the risk of perturbing the other beams. This azimuthal magnetic field results, in the off-axis beams, in a centrifugal radial force which deflects them.
It is known that it is possible, by taking particular care about the configuration of the focuser and of its coil, to reduce the defocusing magnetic components.
It is also possible to contribute to reducing the radial magnetic field by using intermediate pole pieces in the body of the tube.
Improvements may also be made to the gun so that the lines of magnetic flux substantially match the path of the electrons as soon as they are emitted.
It is also possible to vary the inclination of the drift tubes so that they follow the general movement of the beams.
However, all these solutions do not combat the azimuthal magnetic field induced in an off-axis beam by all the other beams.
The object of the present invention is therefore to reduce, or even cancel, this induced azimuthal magnetic field without degrading the gain or efficiency characteristics.
To achieve this, the present invention proposes a multibeam electron tube comprising several approximately parallel electron beams passing through a body. Among these beams, at least some define an interbeam volume. Each of the beams defining the interbeam volume is subjected to a perturbing azimuthal magnetic field induced by all the other beams. The tube includes, in the body, means allowing, in at least one conducting element located in the interbeam volume, flow of a reverse current in the opposite direction to that of the current of the beams, this reverse current generating, in the beams defining the interbeam volume, a magnetic correction field which opposes the perturbing magnetic field.
The conducting element may be incorporated into the body or, on the contrary, electrically isolated from the body.
The means allowing the reverse current to flow in the conducting element incorporated into the body may comprise a ground connection, close to the input of the body, so that the reverse current comes from the current of the beams which is closed by this ground, the collector being at an intermediate potential between that of the cathodes producing the beams and ground.
Preferably, this ground connection is connected to a high-voltage supply which delivers the potential to the cathodes.
In this type of tube, whether for klystrons or traveling wave tubes, the body comprises a succession of cavities and, at the input and output of the cavities, the beams are contained in drift tubes. When the drift tubes are hollowed out within the same conducting block, this conducting block serves as a conducting element in which the reverse current flows.
To force the flow in the interbeam volume, the conducting block may have, in a central part encompassing the interbeam volume, a lower resistance than that possessed by a peripheral part of the block, located around the central part.
To obtain these various resistances, the central part may be made in a first material and the peripheral part in a second material, the second material having the highest resistance.
It is also recommendable to cut chicanes in the perimeter of the periphery of a block in order to increase the resistance at that point.
When two successive cavities have a common wall integral with a conducting block, a resistive insert may be included in the conducting block and the common wall, this resistive insert forcing the reverse current to flow in the conducting block in a loop around the insert and in the common wall on each side of the insert in opposite directions.
The means allowing the reverse current to flow may comprise a first connection means near the input of the body and a second connection means near the output of the body, these connection means being intended to be connected to a supply that has to deliver the reverse current.
In the configuration in which the conducting element is incorporated into the body, the latter and/or the collector must be electrically isolated from various members with which they are normally in electrical contact.
In the configurations in which the drift tubes are not hollowed out within the same conducting block, the interbeam volume is hollow in the drift tubes and it is possible to house therein the conducting element so as to be approximately parallel to the drift tubes and without any electrical contact with the body.
This conducting element may comprise a rigid section at the input and at the output of a cavity and a flexible connection which struddles a cavity while connecting two rigid sections connected on each side of cavity.